Orthogonal Frequency-Division Multiple Access (OFDMA) is a proven access technique for efficient user and data multiplexing in the frequency domain. One example of a system employing Orthogonal Frequency Division Multiplexing (OFDM) is Long-Term Evolution (LTE). LTE is the next step in cellular Third-Generation (3G) systems, which represents basically an evolution of previous mobile communications standards such as Universal Mobile Telecommunication System (UMTS) and Global System for Mobile Communications (GSM). It is a Third Generation Partnership Project (3GPP) standard that provides throughputs up to 50 Mbps in uplink and up to 100 Mbps in downlink. It uses scalable bandwidth from 1.4 to 20 MHz in order to suit the needs of network operators that have different bandwidth allocations. LTE is also expected to improve spectral efficiency in networks, allowing carriers to provide more data and voice services over a given bandwidth. Other wireless standards like WiFi (IEEE 802.11) or WiMAX (IEEE 802.16) also employ OFDM.
OFDM wireless systems transmit a sequence of OFDM symbols that comprise a time transmission interval (TTI).
One advantage of OFDM is the ability to perform frequency-domain equalization of the symbols, which may be performed easily in real time with the use of Fast Fourier Transforms (FFT). However, an open issue in OFDM is how to deal with inter-symbol interference (ISI) and inter-carrier interference (ICI) in an efficient way without impacting the frequency-domain equalization capabilities that make it so attractive. The most common way to deal with ISI and ICI is to reserve a number of samples at the beginning of every symbol containing a repetition of the latest part of the symbol, in order to preserve the cyclicity of the signal and absorb the echoes caused by multipath. The cyclic prefix (CP) does not contain any useful information and hence introduces a loss in efficiency which becomes more important when the symbol length is reduced.
Existing techniques add this cyclic prefix at the beginning of every OFDM symbol. The Cyclic Prefix (CP) must be added to each OFDM Symbol before transmission to accommodate the time spread of the OFDM subcarriers due to fibre dispersion. The CP ensures that the subcarriers remain periodic within the receiver's Fourier Transfer window, to eliminate Inter-Carrier Interference (ICI), caused by phase discontinuities within a window. The CP adds an overhead to the transmission, requiring additional optical bandwidth and reducing receiver sensitivity.
The first OFDM symbol in the TTI usually contains critical control information for the successful decoding of the rest of the symbols, such as scheduling information, pilots for channel estimation and other important control data. Reliable decoding of this first symbol is therefore critical, and a dedicated cyclic prefix appended to the beginning of this first symbol should not be avoided in order to absorb the echoes caused by the channel up to a given maximum delay spread, hence allowing for easy detection.
The current trend in wireless mobile communications is to reduce the end-to-end latencies so as to make the overall system more responsive, and this in general implies a reduction in the symbol duration.
Latency reductions in OFDM directly translate into reductions in the OFDM symbol lengths. The cyclic prefix must be wide enough so as to accommodate the largest delay spread encountered in the scenario, and this puts a lower limit to the size of the CP. Hence, if the symbol length is significantly reduced, the overhead caused by the CP might be unacceptable.
There are alternatives to the use of cyclic prefix (CP) based on the insertion of a number of zeros at the beginning and the end of the information in order to absorb ISI (“Zero-tail DFT-spread-OFDM signals”, G. Berardinelli et al., Proceedings of the 2013 IEEE Global Communications Conference, 2013), but they are only applicable to DFT-spread-OFDM (DFT-s-OFDM).
Other solutions involve estimating the channel impulse response and trying to cancel ISI with an iterative method (“Suppression of Cyclic Prefix in Down-Link LTE like Systems to Increase Capacity”, C. del Amo and M. Fernández-Getino, Proceedings of the 2013 IEEE Vehicular Technology Conference, 2013), but they rely on complex channel estimation procedures that must be linked to the actual ISI interference cancellation algorithms.
Another prior-art solution (“Residual ISI cancellation for OFDM with applications to HDTV Broadcasting”, D. Kim et al., IEEE J. Sel. Areas in Comm., vol 16 (8), 1998) cancels ISI and ICI through an iterative process over all the OFDM symbols, therefore yielding high complexity and also relying on training sequences for channel estimation in the time domain. These training sequences would not be applicable in systems like e.g. LTE, where pilot signals are used instead for channel estimation in the frequency domain.
Another example of existing approaches is described in U.S. Pat. No. 7,606,138, which proposes a particular arrangement of OFDM symbols into a frame by inserting a single cyclic prefix prior to it, thereby increasing spectral efficiency. However the decoding procedure involves multiple direct and inverse Fourier transforms to recover the original samples, as well as complex channel estimation and frequency offset detection procedures. Such procedures rely on particular time-domain training sequences embedded in the signal, and may not be applicable to systems like e.g. LTE where frequency-domain pilots are devoted to channel estimation, or at least involve greater computational complexity at the receiver.
Therefore, there is a need in the state of the art for more efficient ways of dealing with ISI and ICI in OFDM which enable significant symbol reductions in wireless communication systems without compromising the overall efficiency of the system.